The present invention concerns a method for making the open infrastructure of a marine platform buoyant during its conveyance from the builder's yard to the place where it will be immersed and during this immersion-operation.
Marine platforms, those for example which are used for subaquatic oil exploitation, are built close to the shore, at first in a dry dock so that a base for example, a caisson can be erected, and the beginnings of columns, and then on open water in a sheltered site, where the superstructure can be finished during a progressive immersion. Once the construction is finished and the platform put in place, the whole construction is towed to the place provided for its installation, where it is ballasted or immersed by other means and secured on the marine bottom.
During these two phases, on the one hand the towing and the putting in place and on the other hand the full-time utilisation of the platform, the support-structure is subjected to very different and often contrary constraints. During the first phase, the emphasis will be put on good buoyancy of the whole while subsequently it will be good resistance to swell and to bad weather in general which will be the predominant problem.
It is evident that in both phases the structural solidity of the platform will be an essential condition so much so that it will not be an issue subsequently, the constructions envisaged herein are considered to answer the demands for rigidity and durability in all situations, conditions which are now well known in the art.
Among these demands, there is however one in particular that must be raised here; it concerns the resistance of the support-part of the platform which is most exposed to the waves of a swelling sea. On the high seas, the constraints undergone by the infrastructure will be very important and means have already been studied and put into practice which are capable of resisting the strongest swells.
Among these solutions, it has proved particulalry advantageous to mount the platform on a hollow central column surrounded on the periphery by very tall columns, possibly suitably wind-braced. The protection of the structure can take the form of a wall pierced by numerous holes, in particular in the part most exposed to the swell; this wall can also be replaced by annular baulks suitably spaced and intersected with the columns. In the two cases, the exterior structure is normally connected by cross-bars to the hollow central column.
These structures are very interesting from the hydrodynamic point of view, but in order for buoyancy to be achieved, the hollow central column must have had sufficient diameter to be able to serve as floats during towing. Now, after the putting the platform in place, the volume of the interior space of the central column was to a great extent unutilised, the pipe-lines and other useful parts normally passing by this funnel occupying only a small part.
To be able to reduce the diameter of this column to a minimum which is necessary to support the central part of the platform and to allow a passage proportioned to the operational use which is made of it, it was necessary to find another way of increasing the buoyancy of the whole. The problem was equally posed in the case of structures which did not comprise a central column. The aim of obtaining good buoyancy is achieved by the formation of a removable envelope which temporarily surrounds the columns supporting the platform.